netdev - RE: [PATCH] net: mvneta: fix handling of the Tx descriptor counter

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Date:   Wed, 8 Nov 2017 17:03:40 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Simon Guinot' <simon.guinot@...uanux.org>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Sven Müller <musv@....de>,
        Andreas Tobler <andreas.tobler@...udguard.ch>,
        Grégory Clement 
        <gregory.clement@...e-electrons.com>,
        Antoine Ténart 
        <antoine.tenart@...e-electrons.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] net: mvneta: fix handling of the Tx descriptor counter

From: Simon Guinot
> Sent: 08 November 2017 16:59
> 
> The mvneta controller provides a 8-bit register to update the pending
> Tx descriptor counter. Then, a maximum of 255 Tx descriptors can be
> added at once. In the current code the mvneta_txq_pend_desc_add function
> assumes the caller takes care of this limit. But it is not the case. In
> some situations (xmit_more flag), more than 255 descriptors are added.
> When this happens, the Tx descriptor counter register is updated with a
> wrong value, which breaks the whole Tx queue management.

Even with xmit_more it is usually best to limit the number
(in order to reduce tx latency).

OTOH there are probably paths where it 'just happens'.

> This patch fixes the issue by allowing the mvneta_txq_pend_desc_add
> function to process more than 255 Tx descriptors.
> 
> Fixes: 2a90f7e1d5d0 ("net: mvneta: add xmit_more support")
> Cc: stable@...r.kernel.org # 4.11+
> Signed-off-by: Simon Guinot <simon.guinot@...uanux.org>
> ---
>  drivers/net/ethernet/marvell/mvneta.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> index 64a04975bcf8..027c08ce4e5d 100644
> --- a/drivers/net/ethernet/marvell/mvneta.c
> +++ b/drivers/net/ethernet/marvell/mvneta.c
> @@ -816,11 +816,14 @@ static void mvneta_txq_pend_desc_add(struct mvneta_port *pp,
>  {
>  	u32 val;
> 
> -	/* Only 255 descriptors can be added at once ; Assume caller
> -	 * process TX desriptors in quanta less than 256
> -	 */
> -	val = pend_desc + txq->pending;
> -	mvreg_write(pp, MVNETA_TXQ_UPDATE_REG(txq->id), val);
> +	pend_desc += txq->pending;
> +
> +	/* Only 255 Tx descriptors can be added at once */
> +	while (pend_desc > 0) {
> +		val = min(pend_desc, 255);
> +		mvreg_write(pp, MVNETA_TXQ_UPDATE_REG(txq->id), val);
> +		pend_desc -= val;
> +	}

do { ... } while () ??

	David